Neurotransmitter abnormalities in patients with motor neuron disease.

In 22 patients with motor neuron disease (MND), the mean concentration of serotonin (5HT) in platelets was slightly increased, platelet monoamine oxidase (MAO) activity was significantly increased, and plasma concentrations of total and both free and bound tryptophan were significantly decreased. Though platelet MAO activity was positively correlated with concentrations of 5HT, independent causal mechanisms are probable. When patients were rated according to severity, highest values of platelet 5HT and MAO activity were found in the most severely affected group, whereas concentrations of both total and protein-bound tryptophan were most decreased. Changes in concentrations of 5HT and tryptophan may reflect compensatory changes in response to degeneration of motor neurons or to interruption of their monoaminergic innervation.